1. Field of the Invention
The invention relates to an apparatus for removing a surface layer from animal muscular tissue, particularly a skin-including layer from fish fillets, the apparatus comprising a skinning roller driven to rotate and having a circumferential surface designed to grip the material to be treated; a pressure pad provided with a presser surface facing the circumferential surface of the skinning roller, being mounted to be displaced resiliently relative to the circumferential surface of the skinning roller and including a knife receiving pocket; and a blade-like skinning knife received in the pocket.
2. Prior Art
Apparatuses showing these and similar structural features are widely used and are known from printed publications. For example, German Pat. No. 2 049 353 discloses an apparatus, whose structural features correspond to those described above. Use is made therein of a fixed knife blade inserted into the pressure pad, the presser surface being at a limited distance and equidistant from the skinning roller or the circumferentiaI surface thereof and the cutting edge of the skinning knife extending at a distance from the circumferential surface of the skinning roller, which distance corresponds to the thickness of the layer to be severed.
However, as has been found, a satisfactory result could only be obtained when processing the fillets of round fish, this term being used to differentiate from flat fish. The problems encountered in processing fillets of the latter fish type are due to the fact that the skin of such fish is very intimately connected to the muscular substance by a plurality of sinews or similar tendinous ligaments. It was therefore unavoidable to use a drawing cut for processing such fish. In view of this finding and the attempts made to overcome the expensive and maintenance-costly principle of band knife skinners, developments have led towards a system which is characterized by a knife driven to oscillate, as is e.g. disclosed by German Pat. No. 680 720. In this system, the knife is enabled to move from a disengaged and spaced apart basic position into the (closer) working position after the initial portions of the fillet have moved past. Thus, initial fragments are lost and this has to be accepted principally as a production loss.
This deficiency was intended to be removed by the construction in accordance with German Pat. No. 18 10 673, in particular by the embodiment according to FIG. 2 thereof. This construction has an oscillating skinning knife, whose possibilities of pivoting are adjustable and limited in such a way that it remains with a fixedly set spacing with respect to the circumferential surface of the skinning roller. A pressure pad is located below the skinning knife and is at a fixed distance from the circumference of the skinning roller. When using this apparatus, particularly when processing flat fish fillets, a disadvantage arises which is due to the basic concept of this construction. This is revealed in that the tail area splits along the spinal line, so that the skinned fillet receives a dovetail-like appearance, which is considered to be so disadvantageous and an enormous handicap from the quality standpoint that this machine has not been accepted and adopted in this field. The reason for this splitting is the transverse stressing of the fillet during skinning due to the fact that the skinned fillet portions are forced against the oscillating back surface and are accelerated in accordance with the oscillating movement thereof. This effect is supported by that part of the lower surface of the knife which is exposed upstream of the presser surface, the wedge action and the cutting resistance causing an adequately intimate friction between the knife and the fillet, so that the oscillating movement is transferred onto the fillet. In order to reduce this effect by improving the grip of the skin on the skinning roller, the circumferential surface thereof has been provided with a diamond or right-angled knurling. However, apart from a hardly noticeable improvement to the aforementioned effect, this led to a reduction of the reliability of the cutting-start, because now the spacing of the presser surface from the circumferential surface of the skinning roller had to be set at least to the skin thickness of the fillet to be skinned, to ensure that the skin entered underneath the presser surface. However, the resulting gap reduces the pressing action with respect to the skinning roller necessary for a reliable conveying or feeding. In order to enable the necessary close positioning of the presser surface, the circumferential surfaces of the skinning rollers are therefore presently provided with longitudinal grooves, in which the fillet is engaged with its tail end and can thus enter underneath the pressure surface.
Tests carried out with an apparatus according to DE-OS 21 18 164 comprising an oscillating skinning knife which had a reduced moving distance as regards the moving into the working position by the fillet entering into the gap between the skinning roller and the pressure pad also failed to solve the above problem, so that this concept has also not become commercially successful. Apart from the stressing of the fillets as a result of the oscillation, this apparatus led to an above-average unreliability in the initiation stages of the cutting. This is in accordance with the expectations from the aforementioned findings and is mainly due to the fact that the presser surface in its basic position gives the incoming fillet a greater distance from the circumferential surface of the skinning roller than in the working position. Thus, a movement of the pressure pad into the working position only takes place coincidentally, i.e. purely by chance, because for this movement it is necessary that the fillet be adequately entrained by the skinning roller. However, even when the fillet arrives at the knife cutting edge, this entrainment only takes place through friction and pushing engagement of the circumferential surface of the skinning roller roughened by (diamond) knurling or the like. However, this manner of conveying is generally not sufficient to enable the knife to achieve a cutting depth enabling the separated layer to reach the clamping point between the presser surface and the circumferential surface of the skinning roller. However, this is a prerequisite for an adequate torque to become effective on the intermediate gear, in order to bring the knife into the working position close to the circumference and to cause the positive conveying which enables the actual skinning process.